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Assuming an 84% AFUE gas boiler, what will the REAL AFUE be during June-September when I am only needing abuit 2.5M BTU input to make DHW? This is for a 50MBH IBR gas boiler. Any ideas? Seems that the summertime AFUE is going to be kinda low comparted to a chinzy standard tank.

I understand the problem. Combustion efficiency goes down when cycles are shorter. This will effectively lower the AFUE. I know its apples and oranges, so let me rephrase as a statement , you fill in the blank.

For DHW production only, a gas water heater with an EF=??? is the equivalent to a 50MBH IBR boiler that operates at 84% AFUE with a high quality indirect tank.